I've always used the min for min rule although when you're first starting you're pretty inefficient so 1 min in the pool is probably more like 20 min running on the flat. Once you get comfortable, they're pretty even.
Comments:
Have somebody go to the pool with you if you can to check your form. Under the water it should really look like you're running. The most common error is that people get into a bicycling type form, bent at the hips and never really extending the leg out the back. So concentrate on hips directly untder shoulders and pushing out the back with your glutes.
You're right to work up slowly, same as you would if you were returning to an activity on dry land.
Realize that pool running does not work your lower legs much. so when you do go back to doing regular running, you'll still have to ease into it to prevent over working the calves. Especially since you do a lot of hills.
Swimming with a pull bouy can be harder for some people. It makes you work a lot more through the core because you can't use your kick to balance you. This is a good thing. It'll make your stroke more efficient and your kick will go more into propulsion instead of balance when you get back to kicking. It also works your arms a lot so your have to be sensitive about how your shoulders feel. Especially if you tend to have a flatter stroke and don't roll enough.
Living life like there's no tomorrow.
http://gorgebikefitter.com/
2007 Look Dura Ace
2010 Custom Tonic cross with discs, SRAM
2012 Moots YBB 2 x 10 Shimano XTR
2014 Soma B-Side SS